Moscow has more than 300 churches. The oldest one still standing dates back to the early 1400s; the main cathedrals of the Kremlin are about 50 years younger. Those that survived 70 years of Communism, were rebuild afterward or created since represent seven centuries of ecclesiastic architecture. Of course the oldest churches have seen several rounds of renovations, which often resulted in updating their appearance, but there are still a few of great medieval examples around.
For more Russian churches, take a look at Saint Petersburg and its magnificent examples of Baroque, neoclassical and Medieval-revival styles.